fs: encapsulate FSReqWrap more

In further preparation for the Promises enabled fs API,
further encapsulate FSReqWrap details.

The intent here is to isolate, as much as possible, the
functionality of the various fs functions from the details
of how the results are notified. The promises implementation
will use a `FSReqPromise` alternative to `FSReqWrap` that
will use the same API so that both models can be used without
changing any of the actual implementation details for the
various methods.

PR-URL: https://github.com/nodejs/node/pull/18112


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>